[Fleet] Update the install Agent page

[ph]

The instruction page links the user to a download page on elastic.co which hosts the agent installer (which may consist of installer, agent, filebeat, metricbeat, and endpoint). (We also provide instructions for downloading the binary directly from the command line, with warnings that it is less secure).

The user copies and pastes the command and command line options to run the installer. These include a hash of the CA cert for pinning, the enrolment key, the kibana URL, plus various other options.

until we have the MSI installer scoped we can use:

agent enroll -ca_256 <ca_256> http://localhost:5601 <enrol_key>

Require designs/layout.

linked to #55452

[elasticmachine]

Pinging @elastic/ingest (Feature:Fleet)

[ph]

cc @ruflin for awareness, this will be required for the enrolling change.